A better explanation for 19 is that in computer programming, a program will do exactly what you tell it to do. It does not interpret. You have to be careful to avoid telling it to do something you did not intend, because it'll do it. No question.

Sartre's "Being and Nothingness" is a treatise on existence, and non-existence, and what we can say for sure about what exists and what doesn't exist. The coffee-shop is out of cream, but Sartre doesn't want any anyway. So the waitress asks if she can leave out the milk -- which does exist -- in place of the cream.

It's not a hugely intellectual joke -- more like an ironic twist on reverse logic.